Well first of all, as someone who has laid a lot of subway tile, you will be at it a LOOOOONG time with 3x6. I'd do anything you can to increase the tile size to make it go faster. Most 3x6 are lugged to 1/32 or 1/16, so you can lay them without spacers. I don't trust lugging on most brands, it isn't consistent enough on ceramic. I usually space them out to 1/16 and use Powergrout on it because of its low perm rating when fully cured. 4x12 subway is popular and readily available in glazed ceramic. You'll lay 3 per square foot instead of 8. Less than half the work and time, as long as you know how to lay stuff that big without a lip.

Heed the advice to involve your professional tile contractor in whatever you decide. Installing two marble baths right now. One is Calacatta and one is Carrara, both select Italian (see pic - the top is Calacatta, the lower is Carrara). Price varies by format but as an example the 16x16 Calacatta field tile has a trade price of $29/s.f. (no mark-up, no tax, no freight) and $13.60 for the Carrara, 16x16. Another reason to allow your contractor to order it (see photo 2): These are the Carrara moldings (at $37.20/L.F.) that arrived. We expect shade variation in natural stone but 10 of them were outside our tolerated variation (the super light ones on the left). Immediately brought them back to the office, sent a pic to the supplier, who forwarded the pic to the warehouse instructing to carefully select 10 replacements to more closely work with the darker moldings. Arrived the next day. Customer never knew there was an issue.

@palimpsest A lot of additions seemed to be planned from the outside in. Add a 20x20 box on the back and then fit stuff inside it, It might be too big for what you want, it might be two small for what you want. But it seems to be how a lot of people do it. This is so true Palimpsest! I have an acquaintance that did a kitchen addition on her gorgeous 2500SF vintage 1915 craftsman (before she knew me) to make it bigger. Her new kitchen is approx. 14x18 from memory. But it only has one long 18' wall of cabinets with her range, sink and dishwasher all in a row. The addition also has a cathedral ceiling that looks totally out of place in a Craftsman. The kicker: Her refrigerator is till in the adjacent room inside a closet niche. It's just weird that they didn't incorporate the refrigerator in to the new big kitchen area. The other side of the kitchen has a table. To spend that much money to improve a 1915 Craftsman....and to have such an awful layout. I don't get it. I have even seen brand new blueprints where the homeowner's have no clue about furniture placement once their dream home is built. I can visualize space. And they are always shocked when I tell them. All they seem to care about is they have a big new house. Sadly, once they try to move in...they'll have problems placing furniture. Good planning is key! The people in the original post. Seems like their priority is having a bigger space. I just hope they plan that space well and just don't fill it up with stuff. But it's their house.

Ivory, ox blood and beige on the first floor and ivory tall central area and hallways. Sunset color in the entry. Light yellow in DH’s office. Creamy color in guest room and bath. Very light sage color in master bed and bath. Bright sunny yellow walls and angled parts of the ceiling, with a greenish teal on the flat part of the ceiling in my 3rd floor office.